1. Field of the Invention
The present invention relates to a polyester fiber having a core-in-skin composite structure. More particularly, the present invention relates to a polyester fiber having a core-in-skin composite structure exhibiting excellent heat resistance in a rubber matrix and therefore being useful as a reinforcing material for rubber articles.
2. Description of the Related Art
It is well known that various rubber articles are reinforced by polyester fibers, nylon 6 fibers, nylon 66 fibers, or regenerated cellulose (rayon) fibers.
Especially, polyester fibers, typically polyethylene terephthalate fibers, exhibit advantageous physical properties, for instance, high tensile strength, high Young's modulus, low ultimate elongation and low creep and therefore, are adequate as a reinforcing material for the rubber articles. However, conventional polyester fibers are disadvantageous in their inferior bonding (adhering) property to rubber materials compared with conventional nylon 6 or 66 fibers. Therefore, the conventional polyester fibers do not exhibit a satisfactory bonding property to the rubber material as a reinforcing material. Also, when conventional polyester fibers are used as a reinforcing material for the rubber materials, the polyester polymer is sometimes hydrolyzed or aminolyzed with water or an amine contained in the rubber material. This phenomenon causes the heat resistance and bonding strength of the reinforcing polyester fibers in the rubber material to decrease.
Accordingly, it is believed that the heat resistance of the conventional polyester fibers embedded in a rubber matrix is lower than that of nylon 6 or 66 fibers.
Japanese Unexamined Patent Publication (Kokai) No. 56-140128 discloses a rubber-reinforcing material consistiing of core-in-sheath type composite fibers composed of a polyester core constituent and a polyamide sheath constituent, which composite fibers exhibit an enhanced bonding property to the rubber materials.
The above-mentioned type of composite fibers are, however, disadvantageous in that while the composite fiber-reinforced rubber articles are actually used, the polyester core constituent and the polyamide sheath constituent split from each other. This phenomenon results in a poor reinforcing effect of the composite fibers to the rubber material.